Spectrochemical analysis includes a number of techniques for determining the presence or concentration of elemental or molecular constituents in a sample through the use of spectrometric measurements. One particular technique, spectrophotometric analysis, is a method of chemical analysis based on the absorption or attenuation of electromagnetic radiation of a specified wavelength or frequency. A spectrophotometer for providing such analysis generally consists of a source of radiation, such as a light bulb; a monochromator containing a prism or grating which disperses the light so that only a limited wavelength, or frequency range is allowed to irradiate the sample; the sample itself; and a detector, such as a photocell, which measures the amount of light transmitted by the sample.
The near ultraviolet spectral region from 200 to 400 nm is commonly used in chemical analysis. Ultraviolet spectrophotometers usually have at least a lamp as a radiation source, a sensor and appropriate optical components. Simple inorganic ions and their complexes as well as organic molecules can be detected and determined in this spectral region.
In most quantitative analytical work, a calibration or standard curve is prepared by measuring the absorption of a known amount of a known absorbing material at the wavelength at which it strongly absorbs. The absorbance of the sample is read directly from the measuring circuit of the spectrophotometer.
Most gasses have at least one well defined peak of absorption at a certain wavelength. For ozone (O.sub.3), one peak of absorption is at 253.7 nm, in the ultraviolet range of the spectrum. The concentration of a selected gas in a sample can be obtained by solving an equation, known as the Beer-Lambert equation as follows: EQU I.sub.s =I.sub.r *e.sup.-.epsilon.LC
where:
I.sub.s is the intensity of light from the sample; PA1 I.sub.r is the intensity of light from the reference; PA1 .epsilon. is the ozone absorption coefficient constant at the wavelength used; PA1 L is the length of the absorption chamber (path length of the light); and PA1 C is the concentration of gas in weight/volume.
Since L and .epsilon. are fixed quantities, gas concentration can be determined by measuring the intensities I.sub.s and I.sub.r. The Beer-Lambert equation provides an absolute determination of gas concentration. The relationship requires the measurement of a "reference" light intensity and a "sample" light intensity. In known ozone analyzers, the "sample" and "reference" measurements are typically acquired by alternately detecting the amount of light passing through an absorption chamber, cell, or cuvette containing the gas sample, and the amount of light passing through a sample which does not contain ozone.
Several compounds present in certain industrial applications can absorb light at the same wavelength as ozone. When attempting to measure low levels of ozone, these gasses can cause interference or false ozone readings. In many cases the chemical composition of the interfering gasses is unknown or variable. This presents a severe challenge when attempting to detect ozone leaks from either ozone-producing equipment or tubing that conveys ozone required for process steps or ozone produced by a process.
It is known to house ozone generating equipment within a cabinet capable of containing a gas leak. Ambient air, which may include traces of ozone, can be aspirated through a vent and into the cabinet to cool the ozone generating equipment. The warm air from within the cabinet is continuously exhausted to an ozone destruction or neutralization device, such as a scrubber or catalytic converter. Therefore, should the ozone generating equipment develop an ozone leak, the risk of environmental contamination is reduced. Additionally, the functionality of the device that produces the ozone or the process that requires ozone may be compromised.
To further reduce the risk of ozone escaping from the cabinet and to detect equipment failure, it is desirable to detect an ozone leak as soon as possible. However, known devices that merely detect the presence of ozone or which only perform an absolute concentration measurement are unacceptable because air entering the gas containment cabinet may include ozone in an unknown and possibly a varying amount, as well as other possibly interfering compounds. The amount of ambient ozone is unknown because the distribution of atmospheric ozone is not uniform. It is well known that ozone concentration varies from location to location and changes with the temperature and season. The ambient gas concentration is even affected by the presence of electronic and industrial devices. For example, printers and copiers give off ozone during operation. Accordingly, by comparing an gas concentration value to a static reference value, it is impossible to determine whether the difference between the values is indicative of periodically elevated ambient gas concentrations or to an ozone leak.